1. Field of the Invention
This invention relates to controlling the ignition system of an internal combustion engine.
2. Description of the Prior Art
An historic problem with ignition modules has been the large build up of heat at the ignition module output transistor, especially when the engine is at idle and the amount of air flow cooling the ignition module is at a minimum. This heat build up is the result of holding the ignition coil current at a desired current limit for some period of time while waiting for the engine control computer to command the discharging of the coil through the spark plug.
The problem of avoiding excess ignition coil current is further compounded by the lack of accurate engine position information due to infrequent position information input, i.e. twice per cylinder. This lack of information can force the ignition module to start the ignition coil charging sufficiently early so that the coil will have sufficient energy stored to provide an adequate spark irrespective of any change in instantaneous engine rotational velocity. The time during which ignition coil current flows is termed dwell. The time period during which the coil current is at the desired current limit (i.e. excess dwell) can be as much as 40% of the time between successive ignition coil discharges, which at 750 RPM on a 4 cylinder engine is about 16 milliseconds. During this time period the ignition module output transistor will be dissipating approximately 70 watts of power.
Referring to prior art FIG. 1, a graphical representation with respect to time includes waveform A indicating engine rotational position with respect to time, waveform B indicating a signal to provide spark timing, waveform C indicating the time when ignition coil charging begins and the time when the ignition coil reaches coil current limit, and waveform D indicating ignition coil current. In waveform D, the flat portion of the waveform after coil current rises to a maximum current limit is excess dwell.
U.S. Pat. No. 4,303,977 issued to Kobashi et al teaches a method for controlling the current through an ignition coil dependent on the engine speed and magnitude of the supply voltage to the ignition coil. This is an open loop control system which adjusts dwell time based on spark advance and the battery voltage. The resulting current flow is not measured and there is no current feedback to adjust the dwell.
U.S. Pat. No. 4,469,081 issued Mate teaches controlling ignition coil current using a particular transfer function in combination with a plurality of counters. Again, there is no current feedback to adjust the dwell.
U.S. Pat. No. 4,649,888 issued Kawai et al teaches computing a desired spark plug ignition time in accordance with engine load and engine rotation speed, and an energization starting time of an ignition coil in accordance with the ignition timing. The energization starting time is retarded in accordance with the primary current through the ignition coil and the energization time of the ignition coil is reduced. A digital integrator is used to establish the on, or conducting time, of the ignition coil. There is no determination of the coil charge requirement.
U.S. Pat. No. 4,347,570 issued to Akiyama et al teaches an open loop dwell calculation which provides a minimum limit for dwell time and allows spark timing error to occur. The coil current is not measured and there is no feedback of the coil current. As a result, the spark timing is controlled by the dwell. Such a variance of spark timing may be unacceptable where a particular spark timing is desired for optimum engine performance.
U.S. Pat. No. 4,538,585 issued Arguello et al teaches a correction of the next spark event based upon the previous spark event. That is, there is no learned system correction which can be applied. In operation, a current limit adjust window is established for each period. The time of the termination of a dwell in the period relative to the current limit adjust window established for the period starts the dwell in the next period relative to the beginning of the next period at a time calculated to optimize engine performance and minimize energy losses.
In, U.S. Pat. No. 4,519,038 issued to Matsui et al, a reference pulse is generated in response to a predetermined engine crankshaft position. The reference pulse has a leading edge advanced with respect to that predetermined position as a function of the rotational speed of the crankshaft. No feedback of ignition coil current is taught.
U.S. Pat. No. 4,665,884 issued to Yoshida et al teaches an ignition control apparatus arranged to control an ignition timing and a current conduction initiating timing on the basis of a time elapsed from a point in time at which a reference position is reached by a crankshaft. An open loop system is used to control dwell. There is no current limit feedback.
It would be desirable to improve the control of dwell so that excess dwell is reduced. Accordingly, there would be a reduction in the temperature build up within the ignition module by supplying current to the coil only for the period when the coil is charging.